Although light emitting diodes (LEDs) promise long operating life, their static-sensitive nature makes them susceptible to lightning-induced failures. This becomes a significant reliability issue for LEDs used in obstruction warning lights, which may be struck by lightning up to ten or more times a year.
Another reliability concern with respect to LEDs arises when they are electrically connected together in a series network. From an engineering standpoint electrically connecting LEDs in a series network string is desirable since all of the LEDs in the network have the same operating current, thus providing relatively uniform brightness throughout the string of LEDs. One disadvantage, however, is that if just one LED fails open-circuit due to lightning damage, a broken bond wire, a cold solder joint or a bad connection, for example, all of the remaining LEDs in the string will turn off even if they are in operable condition. To overcome this drawback, a “bypass shunt” device such as a zener diode, silicon-controlled rectifier (SCR) or “anti-fuse” is sometimes used in parallel with each LED. Accordingly, if an LED fails open-circuit, a resultant rise in voltage across electrical terminals of the failed LED turns on the bypass shunt device, thereby routing electrical current around the open circuit so that the remaining LEDs in the string that are in operable condition will illuminate.
An example bypass shunt arrangement is shown in FIG. 1. A string or network 10 comprises a plurality of LEDs 12 that are electrically connected in series and are powered by an electrical power supply 14 connected in parallel with the network. Each LED 12 includes a zener diode 16 bypass shunt connected in parallel therewith, the zener diode being reverse-biased with respect to power supply 14. Zener diodes 16 are each configured to have a reverse breakdown voltage that is slightly greater than the forward voltage of a corresponding LED 12, so the zener diode normally remains in a non-conducting or “off” state. However, if an LED 12 fails in an open-circuit state a voltage greater than the reverse breakdown voltage rating of the associated parallel-connected zener diode 16 is present at the terminals of the zener diode, causing it to begin conducting (i.e., switch to an “on” state) so that current supplied by the power supply is maintained in LED series network 10.
A drawback of this arrangement is that, in its conducting state, the electrical power dissipated by zener diode 16 is higher than that of the operational, unshunted LEDs 12. Consequently, heat dissipation considerations must be made for an electronic circuit assembly containing zener diodes 16, such as a printed wiring board assembly, taking into account the potential for a plurality of zener diodes being in a conducting state and dissipating heat at any given time. In addition, zener diodes 16 are physically relatively large devices and thus typically require a significant amount of space on the aforementioned electronic assembly.
With reference to FIG. 2, a more complex string or network 20 comprises a plurality of LEDs 12 electrically connected in series and powered by power supply 14, which is connected in parallel with the network. Each LED 12 includes a silicon controlled rectifier (SCR) 22 bypass shunt connected in parallel therewith. A voltage-sense circuit such as a trigger zener diode 24 or, alternatively, a resistive divider network (not shown) is configured to sense a voltage increase at the electrical terminals of an associated LED 12 when the LED fails to an open-circuit condition and trigger the corresponding SCR to a latched, conducting state. When the triggered SCR 22 is thus latched in its conducting state the voltage drop across its electrical terminals is much lower than the voltage drop of zener diode 16 of FIG. 1 (typically on the order of about 0.8-1.0 Volts) so there is relatively little heat dissipation, even for conditions where somewhat high currents are present in LED string 20. To unlatch the triggered SCR 22 and return it to its non-conducting state the current of LED string 20 must be reduced to a level below the rated holding current for the SCR. Alternatively, the power supplied to LED string 20 by power supply 14 may be momentarily interrupted to return SCR 22 to its non-conducting state. Unlatching a triggered SCR 22 may desirable for situations where an associated LED 12 autonomously resolves its fault, thereby allowing the LED to illuminate.
With reference to FIG. 3 a string or network 30 comprises a plurality of LEDs 12 electrically connected in series and powered by power supply 14, which is connected in parallel with the network. Each LED 12 includes an “anti-fuse” device 32 connected in parallel therewith. Anti-fuse 32 is available from, for example, Murata Electronics North America of Smyrna, Ga. Anti-fuse 32 changes from an off-state resistance of several megohms to an “on” or conducting state having a resistance of a few ohms when the voltage at terminals of the anti-fuse exceeds a predetermined level. Like SCR 22 of FIG. 2, anti-fuse 32 dissipates relatively little power when in a conducting state. However, its relatively small size limits its current-handling capability. In addition, unlike zener diode 16 of FIG. 1 and SCR 22 of FIG. 2, the resistance change of anti-fuse 32 is permanent once placed into a conducting state. Thus, even if a failed LED 12 autonomously resolves its fault the LED will remain bypassed by the associated anti-fuse 32. Furthermore, if an open-circuit failed LED 12 must be replaced, the corresponding anti-fuse 32 shunt must also be replaced, increasing maintenance labor and component expense.
As can be appreciated from the foregoing, although the present art has made some advances in the protection of LEDs in order to increase the overall reliability of LED lighting systems in which they are installed, there remains a need to better protect LEDs that are subject to high voltages due to electrostatic discharge and lightning strikes. This need is particularly great for LEDs that are remotely located or are otherwise relatively inaccessible, such as LEDs used in obstruction lighting.